As is known, when DC current I larger than certain threshold current IC is supplied to a magnetoresistive effect element (MR element) such as a GMR element or IMR element, a spin torque acts on a free layer magnetization in the MR element, thus exciting a steady magnetization oscillation of several to several ten GHz (for example, see non-patent literatures 1 and 2).
This magnetization oscillation is converted into a high-frequency power by an MR effect. A typical element size that allows to observe this phenomenon is about 100 nm×100 nm.
This element is called a magnetic oscillation element since a microwave originates in a magnetic oscillation. Alternatively, this element is also called a spin-torque oscillator since the magnetization oscillation originates in a spin torque.
Use applications of the magnetic oscillation element, which exploit a micro-size oscillator, have been proposed. As one use application, the magnetic oscillation element is used as a reference clock in a portable electronic device.
When a conventional quartz oscillator is replaced by the magnetic oscillation element, not only a space required for the reference clock can be reduced, but also the reference clock can be integrated in a single transmitting and receiving circuit chip. Also, a use application as a local oscillator in a heterodyne detector is available so as to allow an RF spectrum analysis in a single chip.
Since the magnetic oscillation element is also an oscillating field generator originating in a magnetization oscillation, use applications which exploit that oscillating field have also been proposed. One of such use applications is related to a wireless communication, and in this use application, the magnetic oscillation element is applied as a wireless communication transmitter which assumes data transfer between individual components in a computer.
As a use application related to a magnetic recording/playback apparatus, that as an assist recording oscillating field generator is available, as disclosed in, for example, patent literature 1.
A use application which exploits that the magnetization oscillation in the magnetic oscillation element has a response characteristic to an external magnetic field has also be proposed. As a use application related to a wireless communication, the magnetic oscillation element is applied as a wireless communication receiver.
Furthermore, as the use application related to the magnetic recording/playback apparatus, for example, as disclosed in patent literatures 2 and 3, that as a playback head of the magnetic recording/playback apparatus has been proposed.
As principal characteristics required for the magnetic oscillation element common to various use applications, there are three elements, that is, efficient extraction of a high-frequency output from the element, a high output on the μW order, and a stable oscillation frequency.
The reason why the stable oscillation frequency is desirable is obvious in consideration of the use application as a local oscillator and that as a wireless communication transceiver. If the frequency is fluctuating, a role as a reference cannot be assumed, and wireless communication information is disturbed if it is sent at a given frequency.
In order to efficiently extract high frequency from the magnetic oscillation element, it is desired that an element resistance is about 100Ω. This is because many high-frequency devices are designed to have an intrinsic impedance of about 50Ω as a standard, and the magnetic oscillation element as a high-frequency device has to be adjusted to such standard.
Since a typical element size that excites a magnetization oscillation is about 100 nm×100 nm in the magnetic oscillation element, the magnetic oscillation element is required to have 1 Ω·μm2 or less as its RA (a product of the element resistance and a junction area).
The requirement of the RA of 1 Ω·μm2 or less is easily satisfied in a normal GMR element. In a TMR element, although it was impossible to satisfy such requirement in a conventional element, a technique for forming a spacer layer to have a film thickness of about 2 to 3 atoms has been advanced in recent years, and it is possible to set the RA to be 1 Ω·μm2 or less.
On the other hand, in order to obtain a high output on the μW order from the magnetic oscillation element, it is indispensable to enhance the MR effect as the origin of the high-frequency output, that is, to enhance an MR ratio as its index.
As an element with a high MR ratio, a magnetic tunnel junction film having an MgO barrier, that is, a so-called MgO-MTJ, has received a lot of attention in recent years. The MgO-MTJ has an MR ratio as high as several hundred %, and it is well known that an output of several hundred nW can be extracted from a magnetization oscillation in the MgO-MTJ in practice.
However, since MgO is an insulating material, it poses a problem of an dielectric breakdown of an element. As a requirement for an element having a long service life without causing any dielectric breakdown, it is desired that a voltage applied to the element is about 0.1 V or less.
From the above two requirements, current density J of a current to be supplied to the element has to be smaller than 1.0×107 A/cm2 (=0.1 V÷1 Ω·μm2). That is, if J<1.0×107 A/cm2 is satisfied, the two elements, that is, efficient extraction of the high-frequency output from the element and the high output on the μW order, can be solved by exploiting the conventional techniques.
However, a magnetic oscillation element which satisfies the last one element, that is, a stable oscillation frequency, has not been disclosed yet. That is, development of a magnetic oscillation element which satisfies all of the aforementioned three elements and, more specifically, which satisfies J<1.0×107 A/cm2 and has a stable oscillation frequency, is demanded.